1. Field of the Invention
The invention relates to an oscillator having a Schmitt trigger. More specifically, the invention relates to an oscillator having compensation for a response delay of the Schmitt trigger.
2. Description of Related Technology
FIG. 1 illustrates a schematic diagram of a conventional Schmitt trigger oscillator 10. The conventional oscillator 10 includes a charging current source I1, a discharging current source I2, a Schmitt trigger 12, and a capacitor C1, all coupled together as shown. The oscillator 10 provides a square wave output signal Vo2 having a frequency based on the currents supplied by the current sources I1, I2, the value of the capacitor C1, and the logic thresholds of the Schmitt trigger 12.
In general, the charging current source I1 is always on and draws power from a supply Vcc to charge the capacitor C1 with a charging current. When the discharging current source I2 is off, the charging current flows into the capacitor C1 and a charge/discharge voltage Vo1 increases linearly. When the charge/discharge voltage Vo1 crosses a high logic or charge threshold voltage of the Schmitt trigger 12, the oscillator output signal Vo2 transitions to a high level, which turns on the discharging current source I2. Because the discharging current source I2 draws more current than supplied by the charging current source I1, a net charge is removed from the capacitor C1 and the charge/discharge voltage Vo1 across the capacitor C1 decreases. When the charge/discharge voltage Vo1 crosses a low logic or discharge threshold voltage, which is lower than the charge threshold voltage, the output of the Schmitt trigger 12 transitions to a low level, thereby deactivating the discharging current source I2 to begin the charge/discharge cycle again.
FIG. 2 illustrates idealized graphical representations of the input and output signals associated with the Schmitt trigger 12 of FIG. 1. In particular, detail (a) shows the charge/discharge voltage Vo1, which is coupled to the input of the Schmitt trigger 12. As shown by detail (a), the charging period T1 is a function of the charging current and the voltage difference between the charge threshold voltage VB and the discharge threshold voltage VA. Similarly, the discharge period T2 is a function of the net discharge current (I2-I1) divided by the value of the capacitor C1 and the voltage difference between the charge threshold voltage VB and the discharge threshold voltage VA.
Detail (b) of FIG. 2 shows the continuous square wave output signal Vo2 of the Schmitt trigger 12. As shown by detail (b), while the oscillator output signal Vo2 is at a high level, the discharging current source I2 is on and the capacitor C1 is discharging. Additionally, while the oscillator output signal Vo2 is at a low level, the discharging current source I2 is off and the capacitor C1 is charged via the charging current source I1.
FIG. 3 illustrates graphical representations of the charge/discharge voltage Vo1 and the oscillator output signal Vo2 as affected by a charge response delay Td of the Schmitt trigger 12 used in the oscillator 10 of FIG. 1. The charge response delay Td of the Schmitt trigger 12 occurs as the output of the Schmitt trigger 12 transitions between charge and discharge modes of operation. As a result of the response delay Td, the minimum charge voltage of the capacitor C1 undershoots the discharge threshold voltage VA of the Schmitt trigger 12, and the maximum charge voltage of the capacitor C1 overshoots the charge threshold VB of the Schmitt trigger 12. Because the effective charge and discharge threshold voltages have moved apart to VY and VX, respectively, the period of the charge/discharge voltage Vo1 increases and the frequency decreases.
Detail (b) of FIG. 3 shows the oscillator output signal Vo2 that results from the non-ideal charge/discharge voltage Vo1 shown in detail (a). As shown by detail (b) of FIG. 3, the interval during which the oscillator output signal Vo2 is at a low level increases to T1' and the interval during which the oscillator output signal Vo2 is at a high level increases to T2'. The charge response delay Td causes the charging of the capacitor C1 to terminate at the higher voltage VY, which exceeds the ideal charge threshold voltage VB by an amount equal to the charging rate I1/C1 multiplied by the response delay time Td. Similarly, the charge response delay Td causes the discharging of the capacitor C1 to terminate at the lower voltage VX, which is less than the ideal discharge threshold voltage VA by an amount equal to the discharge rate multiplied by the response delay time Td.
The charging interval T1', during which the discharging current source I2 is off, and the discharge interval T2', during which the discharging current source I2 is on, can be expressed as a function of the response delay time Td, the charging current I1, and the discharging current I2, as shown in Equations 1 and 2 below. ##EQU1##
For Equations 1 and 2 above, the value of I2 is assumed to be greater than I1 so that the discharging current source I2 can draw a net charge away from the capacitor C1 while the charging current source I1 is on. As can be seen from Equations 1 and 1, as the value of I2 increases for a given value of I1 the value of T1' increases rapidly to exceed T1+Td and the value of T2' approaches T2+Td. As Equations 1 and 2 demonstrate, even small variations in the response delay time Td of the Schmitt trigger 12 can result in large variations in the charging interval T1', particularly where the discharge rate is relatively high compared to the charge rate. These variations in the charging interval become problematic when using the above-described Schmitt trigger oscillator 10 as a radio frequency oscillator. Furthermore, these problems are compounded significantly in radio frequency oscillator applications requiring a relatively large duty cycle because the response delay time Td has a proportionally larger impact on the control of the charging interval as the desired charging interval time decreases.